Some people have argued that we should have simply tried to deal with Al Qaeda through "police work" -- arresting terrorists when we found them, and not attacking Afghanistan. And certainly not Iraq. So I'm going to speculate how this would have worked.
If we hadn't counterattacked after 9-11, the result would have been that they'd killed about 3000 people for the loss of less than 20, which is a 150 to 1 attrition ratio, even ignoring the facts that the people they killed were each far more valuable than the feces in human form who hijacked the airplanes, and that they'd destroyed two immensely expensive and important buildings.
People jump on winning bandwagons. Over the next months there would have been a huge surge in Al Qaeda recruiting, and of course there would have been follow-on attacks in America and around the world. Why not? We would have shown Al Qaeda that there was absolutely no downside to launching such attacks, and a huge upside in terms of reputation.
While all this was going on, I suppose that we'd be negotiating with the Taliban to extradite "those responsible." The Taliban would have had great fun stretching these negotiations out as long as possible, demanding impossible standards of "proof" of the responsibility of each and every one of them. We would, of course, have to offer those smirking murderers all sorts of foreign-aid incentives in order to "win their cooperation." Every now and then, the Taliban might even agree to turn someone over, only to oh so regretfully report to us that he had fled the country or gone into hiding, oh, so sad, and no of course we're not returning the foreign aid.
Hostile foreign factions, watching America's incredible display of indecisive fecklessness, would lose all their previous inhibitions against attacking on American soil. After all, Al Qaeda did it, and suffered nothing in response. We would have serious terrorist attacks every month, from all over the world. Which would of course convince us that the enemy was too widespread to fight head on; we'd have to offer more foreign aid.
Of course, the sanctions regime against Iraq would have collapsed, and if we weren't going to invade Afghanistan (which actually attacked us) no way would we invade Iraq for merely violating truce terms. I'd give Saddam until no later than 2003 to get his nuclear weapons program restarted; about now he might actually be producing his first warheads.
But at least nobody would be complaining about Abu Ghraib, because there'd be no American jailers putting prisoners in embarassing poses. Instead, Saddam's goons would be raping their relatives and feeding the prisoners into plastic-shredders, which is for some reason that I can't quite grasp morally not as objectionable.
And the Europeans would approve of us more, because we'd have shown them that we could be suckers even more than them.